OPTIMAL FORAGING DECISIONS BY DOGWHELKS, NUCELLA-LAPILLUS (L) - INFLUENCES OF MORTALITY RISK AND RATE-CONSTRAINED DIGESTION

Many animals are capable of ingesting food faster than it can be digested, and the resulting constraint on processing rate may significantly affect foraging behaviour. Dynamic models allow physiological factors, including gut-processing rate, gut fulness and energy reserves to be considered in addition to external influences when predicting foraging decisions. Foraging decisions of Nucella lapillus (L.), a predatory gastropod on rocky shores, were predicted using a dynamic-programming model that included energy reserves and gut contents as state variables, with a limited rate of gut processing. Behaviour was simulated using coupled submodels; an initial choice of remaining in a safe, foodless, refuge or foraging at risk in the open, followed by choices of accepting or rejecting prey during a tidally limited period of searching. The optimal decision to refuge or forage appears to be mainly a function of gut contents; below a threshold level, forage, above, remain in shelter. Because of the gut-contents threshold, dogwhelks should be time minimizers when ingestion rate exceeds digestion rate, but energy maximizers when the converse is true. Energy reserves interact with risk of mortality; high values reducing the amount of time when it is optimal to forage. Smaller dogwhelks are more likely to maintain an energy surplus, perhaps explaining the observed decline in asymptotic size on more risky, wave-beaten shores. The diet is predicted to expand to include prey of lower profitabilities as searching time runs out. Contraction of the diet should occur toward higher ranked prey as the gut-contents threshold for sheltering is approached.